Transition metal compounds computational techniques

There is a growing interest in modeling transition metals because of its applicability to catalysts, bioinorganics, materials science, and traditional inorganic chemistry. Unfortunately, transition metals tend to be extremely difficult to model. This is so because of a number of effects that are important to correctly describing these compounds. The problem is compounded by the fact that the majority of computational methods have been created, tested, and optimized for organic molecules. Some of the techniques that work well for organics perform poorly for more technically difficult transition metal systems. 

Methods for computing the geometric and electronic structures of metal compounds are advancing rapidly on all fronts. While there is a long way to go before Transition Metal computational chemistry can be described as truly predictive, techniques now exist for computing the structures and reactivities of coordination and organometallic systems to useful accuracy. As such, theory can play a powerful complementary role alongside other physical measurements. 

Since organometallic clusters are well defined molecular objects of finite size (although perhaps a bit large), standard molecular quantum mechanical methods like ab initio or density functional techniques are the ideal tools for the description of their electronic structures. Unfortunately, the dimensions of these molecules , combined with the fact that they contain transition metal atoms, places this class of compounds well beyond the range of practical ab initio calculations. Only recently, due to improvements in quantum mechanical computer codes and to the extremely rapid increase in computing power, has it become posable to successfully tackle the electronic structure problem of ligated dusters on a quantitative or semiquantitative basis. So far, most of the theoretical analyses have been based on approximate computational schemes, topological considerations, or even simple empirical rules. 

Transition metal binary carbides represent one of the most studied groups of refractory compounds. They display a unique set of mechanical, thermal and electromagnetic properties, see Toth (1971), Samsonov and Vinitsky (1976), which attract close attention to the peculiarities of their electronic structure and interatomic interactions. As a result, some of the refractory carbides have become classical objects of quantum-chemical investigations and their electronic structures have been studied using almost all computational techniques. 

The papers presented in the conference span the spectrum of activity in the science of alloys. The theoretical presentations ranged in content from fundamental studies of electronic structure, to first-principles calculations of phase diagrams, to the effects of charge transfer, to the temperature dependence of short-range order parameters. They encompassed the study of mechanical properties, the properties of dislocations, of phase evolution, and computer simulations. Experimental studies were presented based on a variety of state of the art experimental techniques, from TEM to synchrotron diffraction. The phenomena studied varied from the precipitation of nitrides in steel, to the wetting of interfaces between two different crystal structures, to the ordering of vacancies in carbides. And the materials whose properties were measured ranged from Transition metals, to the Lanthanides, to the Actinide series of compounds and alloys. 

The most common methods for calculating TM compounds are classical ab initio techniques and DFT. The use of ECPs for describing transition metals and other heavier atoms has become popular in ab initio as well as in DFT calculations, because relativistic effects can be accounted for in a very convenient way and the number of electrons to be computed is drastically reduced. Pseudopotentials were derived originally for use in ab initio calculations. However, it has been shown that ECPs generated from HF calculations may be used in DFT-based methods as well. ... 

Before any computational study on molecular properties can be carried out, a molecular model needs to be established. It can be based on an appropriate crystal structure or derived using any technique that can produce a valid model for a given compound, whether or not it has been prepared. Molecular mechanics is one such technique and, primarily for reasons of computational simplicity and efficiency, it is one of the most widely used technique. Quantum-mechanical modeling is far more computationally intensive and until recently has been used only rarely for metal complexes. However, the development of effective-core potentials (ECP) and density-functional-theory methods (DFT) has made the use of quantum mechanics a practical alternative. This is particularly so when the electronic structures of a small number of compounds or isomers are required or when transition states or excited states, which are not usually available in molecular mechanics, are to be investigated. However, molecular mechanics is still orders of magnitude faster than ab-initio quantum mechanics and therefore, when large numbers of... 

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